The WSDL is also published in the API documentation. On the top at the right side you have the WSDL of the API.
Thanks Christiaan, that is great to hear!
App Services are specifically designed for Mendix 5, and (for now) are based on Web Services/Soap. That may change in the future, we may move towards a Rest implementation (depending on our roadmap priorities).
For now, what you can try, is opening the .msd file in a Zip program (like 7-zip) and take the Wsdl from there, and import it in Mendix 4. But please be aware that we do not really support or encourage this; and it is possible that you will not have full functionality in this way.